The present invention relates to a laminated non-woven fabric for use in a filter and a method for producing the laminated non-woven fabric. In particular, the present invention relates to a laminated non-woven fabric having an excellent filtering efficiency and a long lifetime, and a method for producing such laminated non-woven fabric.
Recently, a non-woven fabric has come to be used as the filtering medium for an air filter, a filter for liquid material, etc. and the production of the non-woven fabric is increasing nowadays.
The non-woven fabric for use in a filter is preferred to have a large pore size at the inlet side (upstream side) and a small pore size at the outlet side (downstream side). To meet this requirement, it has been practiced to laminate a non-woven fabric having a small packing density onto another non-woven fabric having a large packing density, or laminate a non-woven fabric having a large average fiber diameter onto another non-woven fabric having a small average fiber diameter. However, when the non-woven fabrics are laminated with an adhesive, there have been problems such as rapid increasing in filtering resistance at the interface of the laminated fabrics and dissolution of the adhesive into liquid to be filtered. When the filtering resistance increases rapidly at the interface, the pressure loss arisen there causes the peeling of the laminated non-woven fabric layers and accumulation of solid matters, this shortening the lifetime of a filtering medium. Another method of laminating two sheets of non-woven fabric by heat fusing has been proposed. However, the laminated non-woven fabric produced by such a method also suffers from rapid increasing in filtering resistance at the interface.